I believe that the major reason to actively ventilate is to try and
keep the humidity as low as possible inside the trailer. I can't think
of any reason why ventilation would not be used.

Most trailers I've seen with glass gliders inside have these solar
powered fans (I'm in Pennsylvania, where the weather is probably
similar to Central Europe). However, trailers at my gliderport that
contain metal gliders do not have such fans.

As far as trends go, I can only say that my all metal Cobra trailer has
had a solar fan (as well as extra ventilation intakes) for the last 15
years.

-John